The present invention relates to thrust bearings, and more particularly, to uniformly supported, fixed thrust pads for such bearings, which pads have controlled lubrication and groove means for allowing the bearing surfaces to assume friction reducing configurations.
Basically, a thrust bearing is a device adapted to hydraulically transmit axial thrust loads imposed on a shaft to an adjacent stationary support structure. A thrust bearing assembly generally comprises a propulsion shaft having a radial thrust flange secured thereto; a housing enclosing and supporting the shaft with a stationary housing member axially spaced from the thrust flange; and a plurality of thrust pads disposed between the rotating thrust flange and the stationary housing member for transmitting thrust loads therebetween. It is desirable to maintain a hydrodynamic lubricant film between the rotating thrust flange and the bearing surface of the thrust pads to allow the thrust flange to rotate freely, in spaced relationship, over the bearing surface and effectively transmit thrust loads thereto through the lubricating film.
Accordingly, a factor governing effective performance of a thrust bearing assembly and efficient transmission of thrust loads to the thrust pads is the maintenance of the hydrodynamic lubricating film at all operating pressures and temperatures. Recognizing that the thickness of such films are on the order of thousandths and ten-thousandths of an inch, bearing surfaces should be constructed to minimize surface variations under operating conditions so that the bearing surfaces are always separated by a very thin lubricating film. However, the geometry of thrust pad bearing surfaces is appreciably affected by the hydrodynamic lubricant film pressure and differential thermal gradients. For example, the combination of the compressive stress due to the oil pressure in the hydrodynamic lubricant film and the thermal stresses due to differential heating of the bearing surfaces may cause surface contact and failure of the bearing pad, especially near the central portion thereof where temperature and thrust stress conditions are normally the most severe.
Various thrust pad arrangements have been proposed for accommodating the compressive, bending moment and thermal stresses acting on the bearing pads. For example, "pivoted" pad type bearings, as exemplified by U.S. Pat. Nos. 3,423,139; 3,764,187; 3,784,266 and 4,103,979, generally include tiltable segments which are pivotally disposed on rigid support members, such as steel buttons and ribs mounted on the supporting thrust member. Generally, such pivoted pad members are intended by design to tilt on the pivot supports so that a wedge-shaped lubricating film is formed between the rotating thrust member and the thrust pad wherein the film wedge decreases in thickness in the direction of rotation of the thrust member. Although the pivot supports have been offset from the midpoint of the bearing pad to effect efficient tilting thereof, such an arrangement generally has the disadvantage of limiting the shaft to one direction of rotation. Also, although the bearing pads have been made thicker and more complex in construction to strengthen the pad against bearing failure at the pivot point and to cause the bearing surface to assume a particular surface configuration under loading conditions, the very complexity and relative thickness of the bearing pads may cause unpredictable thermal distortions of the bearing surface and localized failure of the bearing pads. Further, the pivotal nature of the bearing pad makes it difficult to construct a central oil passage for maintaining an oil supply to the central portion of the bearing pad.
Other types of bearing pad arrangements include those exemplified by U.S. Pat. Nos. 3,761,151 and 3,966,279, wherein the bearing pads are hydraulically supported, and U.S. Pat. No. 3,930,691 which relates to a "swing" pad bearing.